Generally, agricultural machines comprise elements that can be adjusted with respect to the frame or chassis of the machine by means of associated actuators—that is, they can be linear displaced or pivoted. In the case of a tractor, these elements can comprise a work device that is placed on a front or rear power lift of a tractor, for example, for the processing of the soil, for sowing, or to apply fertilizers or other chemicals—in the case of a self-propelled field sprayer, a height-adjustable spray rod and/or one which can pivot around an axis that extends in a fore-and-aft direction; with a harvester or a field chopper, a harvesting attachment that can move up and down by pivoting around a horizontal axis that extends transverse to the forward direction (generally, this horizontal axis extends through a rotating axle of an upper inclined conveyor roller or chopping cylinder) and/or for the parallel alignment on a lateral incline, around which an axis extending in the forward direction can be swiveled, or an ejection spout of a forage harvester that can be used to adjust the height of its output end around a horizontal axis and/or to adjust the angle around a vertical axis.
The theoretical or nominal (i.e. the commanded or desired) position of the actuator, and thus the commanded or desired position of the adjustable element, is selected by an operator. The operator makes the selection using a suitable operator interface or by means of an automatic control. Thus, for example, the operator can directly command the cutting unit of a harvester to move to a particular height above the ground with an operator interface. Alternatively, the commanded or desired position of the actuator may be provided with the aid of the actual position of the harvester, from a map, in which the cutting unit heights used during the last harvest have been entered (see e.g. DE 44 31 824 C1) in association with a particular location on the ground. As yet another alternative, the commanded or desired position of the actuator, may be generated using the aid of predictive sensors (see e.g. EP 1 269 823 A1) that predict the desired height above the ground.
In these typical prior art cutting unit height control systems, the actual height of the cutting unit above the ground is detected by means of a sensor. The control unit controls the actuator to adjust the height of the cutting unit with respect to the harvester by minimizing the difference between the commanded or desired height and the actual height detected by means of the sensor. Typically, a control unit producing a signal proportional to the sensed height error is used for this. Control units configured in this manner are said to use “proportional control” or to be “proportional controllers”.
When the actuator changes the position of the cutting unit (for example, by raising the cutting unit or lowering it, or by lateral tilting it), the sudden movement of the cutting unit gives rise to excitatory moments and forces. These moments and forces are transferred to the frame or chassis of the harvesting machine. Since the harvesting machine is typically supported on large-volume tires or elastically suspended rubber belt tracks, the system (consisting of the harvesting machine and the cutting unit) is excited to mechanical vibrations (e.g. to bounce up and down above the ground or twist back and forth along its longitudinal axis). This bouncing or twisting negatively affects the accuracy of the position of the height and the lateral inclination of the cutting unit.
Therefore, with such control units employing proportional control, it is desirable to select control unit parameters. In particular, the control unit amplification, which determines how suddenly the actuator reacts to a given height error (i.e. the difference between the commanded or desired height and the real height) in such a way that the control unit behavior is optimized. By selecting proper control unit parameters, the actual height is adjusted, as quickly as possible, to the commanded or desired height, without producing excess vibrations or bouncing of the harvesting machine and/or cutting unit.
In the prior art, the control unit amplification parameter is manually entered by the operator into the control unit. A suitable interface is provided to the operator for entering the control unit amplification, in particular, a rotary knob, or with a value based on experience, it is definitely specified (see e.g. Y. Xie et al., “Fundamental Limits in Combine Harvester Header Height Control”, Journal of Dynamic Systems, Measurement, and Control, May 2013, 0345031-0345038).
The response characteristics of the control unit, however, are dependent on a number of additional parameters of the system in addition to the control unit amplification, such as soil characteristics, parameters of the tires, the grain tank fill state, parameters of the harvesting attachment, parameters of the hydraulic system, parameters of the machine, etc.). The manual (i.e. operator) solution of adjusting the control unit amplification requires regular adjustment of the control unit amplification in order to accommodate these other parameters of the system as they change over time, traveling through a field. To accommodate these additional parameters of change over time the operator would have to regularly adjust the control unit amplification to attain a compromise among the reaction time, harmonic range, stability, and robustness on the basis of the changing circumstances during operation in an agricultural field.
Typically, however, the operator adjusts the control unit amplification only once for the whole day, which is then followed by a strong performance loss as the machine and field conditions change over time. This degradation in performance may be so extreme that the operator is forced to reduce the machine's harvesting speed (i.e. the machine's speed over the ground) to prevent the vibrations (bouncing). This speed reduction impairs the harvesting performance of the machine and reduces its productivity.
DE 10 2014 203 005 B3 proposes superimposing a time-delayed signal onto the adjustment signal for the height control of an agricultural harvesting attachment, sent by a specification device to an actuator based on theoretical and real values of the height of the cutting unit, in order to diminish the aforementioned vibrations. The amplification and time delay of an amplifier making available the time-delayed signal is determined by means of a determination device, which, before the beginning of the harvesting process and/or during the operation, causes the extension of the actuator for the adjustment of the height of the harvesting attachment.
With the aid of signals provided by a pressure sensor associated with the actuator or provided by an inertia sensor on the cutting unit, the determination device determines the resonant frequency (i.e. the natural frequency of vibration or bouncing) of the harvesting attachment and harvesting machine and determines the appropriate damping of the system, and, with this assistance, the amplification and time delay of the amplifier that makes available the time-delayed adjustment signal. Its amplification can also be specified by the determination device. In this arrangement, therefore, only the adjustment signal supplied to the actuator is modified, and not the adjustment signal made available by the specification device that is used as a control unit.
U.S. Pat. No. 7,707,811 B1 describes a self-propelled swath mower with a front mowing unit and an automatic contact pressure control unit associated with it. In order to determine the weight of the mowing unit, which is needed for the correct control of the contact pressure, the contact pressure control unit investigates whether the mowing unit is in a suitable position so as to determine the pressure in the lifting cylinder of the mowing unit. The position is considered to be suitable if the mowing unit is at a relatively great height above the ground—for example, upon turning in the headland or during road travel. Therefore, only a static parameter of the mowing unit (weight force) is determined here. This control unit is suitable for the adjustment of the contact pressure, but not for the control of the height of a harvesting attachment or another element, since other parameters which characterize its movement behavior have to be determined for this.
EP 0 331 893 A2 describes a combine harvester with a control unit device for the control of the height and the swivel angle of the cutting unit around an axis which extends in the forward direction. The actual height of the cutting unit above the ground is determined by the ground scanner and, based on the difference between the theoretical and actual values of the height of the cutting unit, a microprocessor controls actuators for the adjustment of the aforementioned angles. The hysteresis range and the dead time of the theoretical-to-actual comparison of the control unit device and changes of the control unit characteristics thereby produced are derived from the reactions of the control unit device, wherein the periodicity of the adjustment signals and perhaps the vicinity of the boundary load can be used, and depend on the work conditions, such as the traveling speed, the ground unevenness, and the cutting height. In this way, proportional-differential-integral control units with dynamic parameter adaptation will be attained. Accordingly, control unit parameters in the form of hysteresis and dead time are improved here (iteratively), in that the adjustment signals of the control unit device are detected. The procedure is disadvantageous because the dead time and hysteresis represent only a few of the parameters of the control unit device to be appropriately controlled. It is also disadvantageous because the iterative procedure is based exclusively on the detection of the adjustment signals of the control unit device, which works very indirectly, in a time-delayed manner and is thus subject to errors.
The goal of the invention under consideration is to provide an arrangement for controlling an actuator that adjusts an adjustable element of an agricultural work machine in which the aforementioned disadvantages are entirely or at least to some extent, avoided.